Digital radiography (DR) systems are enjoying growing acceptance in medicine and industry, with particular value as clinical imaging tools. As shown in the simplified block diagram of FIG. 1, radiation from a radiation source 12 in a DR imaging apparatus 10 is passed through a subject 14 and impinges upon a radiation detector 30 that includes a scintillator screen 16 for converting the energy from ionized radiation into light radiation having a different frequency, typically within the visible spectrum, and an image sensing array 20. Image sensing array 20, typically mounted on the backplane of scintillator screen 16 or otherwise optically coupled with scintillator screen 16, then forms a digital image from the emitted light that is excited by the incident radiation. The digital image thus formed can then be processed and displayed by an image processing apparatus on a control logic processor 18, typically provided by a computer workstation and display monitor.
Unlike conventional X-ray film apparatus, DR imaging apparatus 10 does not require a separate processing area, light-protected environment, or image processing consumables. Another advantage of DR imaging technology is speed, since images are obtained immediately after the X-ray exposure. For medical applications, this means that a diagnostic image can be provided to medical personnel while a patient is still present at an imaging facility.
Image sensing arrays 20 for radiographic applications typically consist of pixel sites, commonly referred to as pixels, each pixel having a photo-activated image sensing element and a switching element for reading a signal from the image-sensing element. Image sensing can be performed by direct detection, in which case the image-sensing element directly absorbs the X-rays and converts them into charge carriers. However, in most commercial digital radiography systems, indirect detection is used, following the basic arrangement shown in FIG. 1, in which an intermediate scintillator element converts the X-rays to visible-light photons which are then sensed by a light-sensitive image-sensing element.
Examples of image sensing elements used in image sensing arrays 20 include various types of photoelectric conversion devices such as photodiodes (P-N or PIN diodes), photo-capacitors (MIS), or photoconductors. Examples of switching elements used for signal read-out include MOS transistors, bipolar transistors and p-n junction components.
FIG. 2 shows an arrangement of components in a conventional image-sensing array 20 used for DR imaging. In one basic embodiment, a pixel 22 consists of at least one photoelectric conversion device or photosensing (PS) element 24, shown as a photodiode in FIG. 2, and at least one switching element 26, shown as a type of transistor switch, MRO. Operation of image sensing array 20 consists in the steps of (a) initializing the array of pixels 22, (b) exposing the array to the light radiation excited by X-rays and (c) reading the signal value at each pixel of the array using a multiplexed signal-reading sequence.
As an example of switching element 26, transistor MRO is addressed by a gate line driven by a signal ΦRO. Each data line, in turn, is connected to an external charge amplifier (not shown), as is familiar to one skilled in the imaging electronics art. During signal integration, switch MRO is off and photosensing element 24 integrates the photocurrent generated by external light, with added dark current thermally generated within the photo-sensor. During readout, MRO is switched on, one row at a time, transferring the charge from photosensing element 24 to the data line, where it is sensed by a charge amplifier at the end of the column.
One problem with existing embodiments of image sensing array 20 relates to the amount of time required to obtain an image. Read-out of array 20 can take a full second or longer, due to a number of factors. Each array 20 provides a large amount of data, typically from 3,000×3,000 pixels, each pixel 140 microns square in a typical embodiment. There is a relatively long gate address time and long data settling time for each pixel. At the analog-to-digital data conversion end, high accuracy is required, typically in the range of 14-bit resolution.
Another recognized problem with image sensing array 20 in conventional embodiments relates to a disappointing signal-to-noise ratio. One of the largest noise sources in traditional arrays is photosensor dark current. The dark current within a photosensor can be due to thermal generation of electron-hole pairs or, at high bias voltages, to electric field-induced breakdown. The dark current produces an offset in the pixel signal which frequently must be subtracted from the image through frame-to-frame captures and digital subtraction. Unfortunately, this offset, that varies with the type of photoelectric conversion type used and can vary with integration time, can often be larger than the actual image signal level in radiographs. In addition, the dark current results in noise generated from other sources. These noise sources include quantum noise, 1/f noise or flicker noise and pattern noise.
The dark current shot noise (in electrons) is given by:N=(JD*(Tint+Tro)/q)1/2 where JD is the photosensor dark current, Tint+Tro are the signal integration and readout times respectively, and q is the electronic charge. The pattern noise is given byN=α*JD*(Tint+Tro)/q where α is the percentage rms variation in pixel-to-pixel dark current level. The pattern noise is usually subtracted by capturing multiple dark frames before or after the radiographic image capture and digitally subtracting the averaged dark frames from the image frame. This subtraction process adds noise due to digitization and other electronic noise sources.
The 1/f noise is given by:N(f)=(β*JD*(Tint+Tro)*(f/fo)/q)1/2 
Since the readout time is often more than 10 times longer than the light integration time, the dark current and the resulting shot noise, pattern and flicker noise is predominantly generated during the readout. Provision of a low noise storage element in the pixel would allow the signal charge to be stored without being degraded by noise and offset from the pixel.
Some radiographic imaging modalities, such as fluoroscopy or image-guided surgery, require video-rate imaging. For these applications much lower conversion accuracy and higher-speed readout electronics are used, at the price of reduced signal-to-noise (S/N) ratio. Reduced S/N ratio may be acceptable in such cases. However, there are also a number of radiographic modalities, such as multi-energy, CT or cone-beam CT, that require capture of a sequence of images having the best possible resolution and overall image quality. Currently, because of the long readout time of conventional arrays, the sequence used for such imaging requires the patient to be immobile for several seconds while the successive frames are captured and then read out. Inadvertent movement of the patient during imaging would require repetition of the imaging sequence, exposing the patient to increased radiation dosage and requiring additional time and cost.
There have been a number of proposed solutions for reducing the read-out time required for an image sensor array. For example:                U.S. Pat. No. 6,429,436 (Tomisaki et al.) describes an array panel having photodetectors with signal lines routed on both sides of the panel to reduce parasitic capacitance and allow multiple simultaneous read operations.        U.S. patent application Publication No. 2005/0173645 (Endo) describes a metal-insulator-semiconductor (MIS) structure having reduced frame-by-frame wait periods.        
There have also been solutions proposed for improving signal quality and for overall noise reduction using additional switching and signal storage components. As an example, U.S. patent application Publication No. 2005/0018065 (Tashiro et al.) describes a pixel readout configuration for an image array in which a holding capacitor is switched between a sampled signal and an output line.
While these and similar solutions have been proposed for improving the overall signal quality and response time of an imaging array panel, however, other imaging problems related to signal-to-noise ratio have not been adequately addressed. Dark current noise, which degrades S/N performance, is still a factor with existing solutions. Other problems, such as array panel storage of multiple successive images obtained at near-video rates, have not yet been addressed.